1. Field of the Invention
The present invention relates to an ultrasonic receiving apparatus for receiving ultrasonic waves, and further to an ultrasonic imaging apparatus to be used for medical diagnosis or nondestructive inspection by receiving ultrasonic waves using such an ultrasonic receiving apparatus.
2. Description of a Related Art
Conventionally, in an ultrasonic imaging apparatus, generally a one-dimensional sensor array using a piezoelectric element including a piezoelectric ceramic represented by PZT (Pb (lead) zirconate titanate) or a macromolecule piezoelectric element such as PVDF (polyvinyl difluoride) has been used as an element (vibrator) for transmitting and receiving ultrasonic waves. Two-dimensional images in plural cross sections of an object to be inspected are obtained while mechanically shifting a one-dimensional sensor array as described above, and further, by synthesizing these two-dimensional images, a three-dimensional image is obtained.
However, according to this technique, since a time lag is generated in the shifting direction of the one-dimensional sensor array, cross-sectional images at different time points are synthesized resulting in a blurred synthesized image. Accordingly, the technique is not suitable to such a case where images of a living organism as an object are taken in ultrasonic echo observation or the like.
In order to obtain high quality three-dimensional images using ultrasonic waves, a two-dimensional sensor capable of obtaining two-dimensional images without shifting the sensor array is required.
However, although minute processing on elements and wiring to a number of minute elements are required in the case where the two-dimensional sensor array is manufactured using the above-described PZT or PVDF, further miniaturization and integration of elements exceeding the state of the art are difficult. Also, even when the above-described problems are solved, such problems still remain that the cross talk between elements is increased, the SN-ratio is lowered due to increase of electrical impedance caused from minute wirings, electrodes of minute elements get damaged easily, and so on. Therefore, it is difficult to achieve the two-dimensional sensor array using the PZT or the PVDF.
On the other hand, another type of sensor is also known, in which received ultrasonic wave signal is converted into an optical signal and then detected. As for a photo-detection type ultrasonic sensor, a sensor in which a fiber Bragg grating (abbreviated as FBG) is used (see TAKAHASHI et al., National Defense Academy xe2x80x9cUnderwater Acoustic Sensor with Fiber Bragg Gratingxe2x80x9d, OPTICAL REVIEW Vol. 4, No. 6 (1997) p. 691-694), and a sensor in which a Fabry-Perot resonator (abbreviated as FPR) structure is used (see UNO et al., Tokyo Institute of Technology xe2x80x9cFabrication and Performance of a Fiber Optic Micro-Probe for Megahertz Ultrasonic Field Measurementxe2x80x9d, T.IEE Japan, Vol. 118-E, No. 11, ""98) are reported. When a two-dimensional sensor array is manufactured by using an ultrasonic sensor as described above, the following advantages can be obtained, that is, electrical wiring to a number of minute elements is not required and satisfactory sensitivity is obtained.
Further, a photo-detection type ultrasonic sensor having a two-dimensional detection surface is also known. For example, Beard et al., University College London xe2x80x9cTransduction Mechanisms of the Fabry-Perot Polymer Film Sensing Concept for Wideband Ultrasound Detectionxe2x80x9d, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 46, NO. 6, NOVEMBER 1999 discloses that a polymer film having a Fabry-Perot structure is used for detecting ultrasonic waves. In a film-like ultrasonic sensor as described above, the cost can be reduced since processing on a number of minute elements is not required.
However, the photo-detection type ultrasonic sensor has the following problem, that is, multiple reflection of an ultrasonic wave is generated on a backside of the ultrasonic wave receiving surface.
Herein, taking a photo-detection type two-dimensional plane sensor as an example, the multiple reflection of an ultrasonic wave will be explained. As shown in FIG. 15, an ultrasonic detecting element 100 includes a substrate 101 and an ultrasonic sensing portion 102. In this example, the ultrasonic sensing portion 102 has a Fabry-Perot resonator structure including a total reflection mirror 103, a half mirror 104 and a cavity 105 being formed between the total reflection mirror 103 and the half mirror 104. The member forming the cavity 105 is subjected to a geometrical displacement by being applied with an ultrasonic wave.
While allowing light to enter into the ultrasonic detecting element 100 from the substrate 101 side, an ultrasonic wave is applied to a receiving surface 102a of the ultrasonic detecting element 100. Then, owing to the acoustic pressure changes of the ultrasonic wave, the optical path length L of the cavity 105 changes in accordance with the position of the receiving surface 102a, and the light intensity reflected from the ultrasonic sensing portion 102 changes corresponding to the position thereof. By converting the intensity of the reflected light into the intensity of the ultrasonic wave, the intensity of the ultrasonic wave, which corresponds to the position of the receiving surface 102a, can be detected.
Referring to FIGS. 16 and 17A, the ultrasonic wave propagating from medium and containing information concerning an object to be inspected generates vibration at a point A, and propagates into the inside of the ultrasonic detecting element 100 (ultrasonic wave US1). Then, the ultrasonic wave US1 is reflected at a point B of an interface on the opposite side of receiving surface 102a. At this moment, the ultrasonic wave US1 generates vibration at the point B and returns toward the direction of the receiving surface 102a (ultrasonic wave US2). Further, the ultrasonic wave US2 is reflected at the receiving surface 102a. At this moment, the ultrasonic wave US2 generates vibration at a point C and propagates again to the rear surface of the receiving surface 102a (ultrasonic wave US3). Thus, in the ultrasonic detecting element 100, the reflection is repeated until the propagated ultrasonic wave fades away. Owing to this phenomenon, as shown in FIG. 17B, the signal from the ultrasonic detecting element 100 is mixed with signals generated through the multiple reflection (detection signals at the points C and E) in addition to the signal concerning the object to be inspected (a detection signal at the point A) that is to be normally detected.
The above described multiple reflection of the ultrasonic wave becomes a cause to decrease the SN-ratio in the ultrasonic image, resulting in a decreased image quality. Accordingly, for example, in an ultrasonic receiving apparatus that uses piezoelectric element for transmitting and receiving ultrasonic waves, the ultrasonic wave is attenuated by connecting a backing material including a ferrite core or the like to a piezoelectric element. However, in the photo-detection type ultrasonic receiving apparatus, since it is necessary to take the optical transmissibility into consideration, the backing material same as that of conventional manner can not be used.
The present invention has been achieved in view of the above-described problems. An object of the present invention is, in a photo-detection type ultrasonic receiving apparatus, to increase the quality of the ultrasonic image by reducing the multiple reflection of the ultrasonic wave. A further object of the present invention is to provide an ultrasonic imaging apparatus to be used for medical diagnosis or nondestructive inspection by receiving ultrasonic waves using such an ultrasonic receiving apparatus.
In order to solve the above-described problems, an ultrasonic receiving apparatus according to a first aspect of the present invention comprises an ultrasonic detecting element for modulating light on the basis of a received ultrasonic wave; a backing portion, directly or indirectly connected to the ultrasonic detecting element, for propagating the ultrasonic wave received by the ultrasonic detecting element, the backing portion having optical transmissibility and guiding the light used for detecting the ultrasonic wave; and a photoelectric conversion unit for detecting the light output from the ultrasonic detecting element.
Further, an ultrasonic receiving apparatus according to a second aspect of the present invention comprises an ultrasonic detecting element including an ultrasonic sensing portion which is expanded and contracted by a received ultrasonic wave to change an optical reflectance in accordance with expansion and contraction thereby performing intensity modulation of incident light; an optical transmission path for guiding the light to the ultrasonic detecting element and propagating the ultrasonic wave received by the ultrasonic detecting element; a collimating portion for collimating the light guided by the optical transmission path with respect to the ultrasonic detecting element; and a photoelectric conversion unit for detecting the light reflected from the ultrasonic detecting element.
An ultrasonic imaging apparatus according to a first aspect of the present invention comprises an ultrasonic transmitting unit for transmitting an ultrasonic wave in accordance with a drive signal; a drive signal generating circuit for generating the drive signal to be applied to the ultrasonic transmitting unit; an ultrasonic detecting element for modulating light on the basis of a received ultrasonic wave; a backing portion, directly or indirectly connected to the ultrasonic detecting element, for propagating the ultrasonic wave received by the ultrasonic detecting element, the backing portion having optical transmissibility and guiding the light used for detecting the ultrasonic wave; and a photoelectric conversion unit for detecting the light output from the ultrasonic detecting element.
Further, an ultrasonic imaging apparatus according to a second aspect of the present invention comprises an ultrasonic transmitting unit for transmitting an ultrasonic wave in accordance with a drive signal; a drive signal generating circuit for generating the drive signal to be applied to the ultrasonic transmitting unit; an ultrasonic detecting element including an ultrasonic sensing portion which is expanded and contracted by a received ultrasonic wave to change an optical reflectance in accordance with expansion and contraction thereby performing intensity modulation of incident light; an optical transmission path for guiding the light to the ultrasonic detecting element and propagating the ultrasonic wave received by the ultrasonic detecting element; a collimating portion for collimating the light guided by the optical transmission path with respect to the ultrasonic detecting element; and a photoelectric conversion unit for detecting the light reflected from the ultrasonic detecting element.
According to the first aspect of the present invention, since the ultrasonic wave is attenuated by propagating the received ultrasonic wave to the backing portion, it is possible to avoid the influence due to the multiple reflection of the ultrasonic wave. Also, since the light used for detection is guided by the backing portion having an optical transmissibility, it is possible to reduce the attenuation of the optical signal output from the ultrasonic detecting element and to guide the optical signal to the photoelectric conversion unit without decreasing the SN-ratio.
According to the second aspect of the present invention, it is possible to avoid the influence due to the multiple reflection of the ultrasonic wave by propagating the received ultrasonic wave to the optical transmission path for guiding the light to the ultrasonic detecting element. Also, since the ultrasonic detecting element and the optical transmission path are connected via collimating portion, it is possible to guide the parallel light to the ultrasonic detecting element and to propagate the ultrasonic wave to the optical transmission path such as an optical fiber.